The present invention relates to an active matrix type of liquid crystal display device.
In general, in a liquid crystal display device, an electric field is applied to liquid crystal molecules of a liquid crystal layer interposed between a pair of substrates to change the alignment direction of the liquid crystal molecules. A display produced by the liquid crystal device results from a selective change of the optical characteristic caused by a change in the alignment direction of the liquid crystal molecules.
The typical conventional active matrix liquid crystal display device which operates in a twisted nematic display mode (TN) has electrodes which are arranged on each of a pair of substrates having liquid crystal material disposed therebetween, and the direction of electric field applied to the liquid crystal material is set so as to become nearly normal to the substrate interface, whereby a display is produced by using the optical rotary power of the liquid crystal material. The most important problem in a liquid crystal display device operating in the TN display mode is that is has a narrow view angle.
On the other hand, U.S. Pat. No. 4,345,249, WO91/10936 discloses a liquid crystal display device which operates in the in-plane switching (IPS) mode, in which an electric field having a component nearly parallel to the substrate surface is produced using interdigital electrodes so as to cause rotational movement of the liquid crystal molecules nearly within a plane, whereby a display is produced using the birefringence of the liquid crystal material. The IPS mode is caused by in-plane switching of the liquid crystal molecules, and it has the advantages of providing a wide viewing angle and low storage capacitance compared to the conventional TN mode. Therefore, the IPS mode is a promising mode of operation for a new liquid crystal display devices, replacing the TN mode, and it is one of technologies which has seen rapid progress in recent years.
Further, Japanese Patent Application Laid-Open No. 9-73101 discloses a liquid crystal display device operating in the IPS mode in which at least one of the electrodes to be supplied with a voltage is formed of a transparent conductive film to improve the transmittance. A bright liquid crystal display device having good viewing characteristics (brightness contrast ratio, gray scale reversal) as described above has potential for application to a monitor or a television set having a large display area.
However, although the IPS mode type of liquid display device has a good display performance, as described above, display defects such as display unevenness, a sticking image referred to as an after image, and the like, sometimes appear when it is used for long time. Particularly, the sticking image or after image is recognized as a phenomenon that occurs when one pattern is displayed for a long time, and after that, when another pattern is displayed, the former pattern is displayed at the same time. It is also recognized that this problem is one of the factors which serves to degrade the liquid display performance.
The cause of the after image phenomenon is not clear yet, but the display defect of the after image and sticking image in the conventional TN mode type of liquid crystal display device is estimated to be caused by the fact that a direct current charge is accumulated in the interface of the liquid crystal alignment layer of each pixel because absorbing of ionic components to the alignment layer itself or to the surface of the alignment layer, which effect a change in the effective voltage. That is, the display defect of after image and sticking image is believed to be caused by the fact that the electric potential of the applied voltage is not eliminated within the response time, but is held and remains in the alignment layer of the pixel electrode or in the interface between the liquid crystal and the alignment layer, which changes the effective voltage.
In the TN mode, the correlation between the after image phenomenon and the residual DC voltage described above has been studied, and it has become gradually more clear that the problem of the after image phenomenon can be solved by reducing the residual DC voltage. Therefore, the alignment layer of the conventional TN mode requires a property in which the DC charge is hardly accumulated, that is, an alignment layer having a small residual DC voltage component. For example, in the technology relating to the TN mode type of liquid crystal display device as disclosed in Japanese Patent Application Laid-Open No. 8-54631, by considering that the after image is caused by absorption of ionic impurities in the liquid crystal into the alignment layer due to a generated driving DC voltage which accumulates the electric field, a method of using a low resistance alignment layer has been proposed in order to moderate the electric field accumulated in the alignment layer.
On the other hand, in the IPS mode, the following technologies for suppressing the after image have been proposed based on a cause of the after image similar to that of the TN mode described above. For example, in the technology disclosed in Japanese Patent Application Laid-Open No. 7-181439, by considering that the after image in the IPS mode is caused by a dipole of the liquid crystal molecules inducing polarization in the alignment layer, there has been proposed a method of suppressing the after image by setting the dipole moment of a side group in the liquid crystal molecules to a value below 3 Debye in order to reduce the polarization induced in the alignment layer. Further, in the technology in regard to an IPS mode liquid crystal display device as disclosed in Japanese Patent Application Laid-Open No. 7-159786, in order to rapidly modify the charge due to polarization produced in the liquid crystal layer, the alignment layer and the insulation film, there has been proposed a method of using an alignment layer and an insulation film having a surface resistance value within 3.3×1011 to 2.5×1018 Ω/cm2, and there also has been proposed a method of reducing the after image by specifying a relative relationship of relaxation times expressed by a dielectric constant and a specific resistance of each of the liquid crystal material, the alignment layer and the insulation film. Further, in the technology in regard to an IPS mode type of liquid crystal display device as disclosed in Japanese Patent Application Laid-Open No. 10-123526, there has been proposed a method of reducing the after image by using an alignment layer having a high ion adsorption in the substrate opposite to the substrate on which a group of electrodes for applying an electric field to the liquid crystal is formed.
There is a correlation between the after image phenomenon and the residual DC voltage in the conventional TN mode, as described above. The correlation was studied for the IPS mode, and the following results were obtained.    (1) There was little significant difference in the residual DC voltage between a liquid crystal display element not causing the after image and a liquid crystal display element causing the after image.    (2) In the IPS mode, there were some liquid crystal display elements in which sticking image semipermanently continued, and the dark level was decreased, and the contrast was substantially decreased.
It was also clarified from investigation of the alignment direction of liquid crystal in the regions of the after image and sticking image that some alignment directions were rotated from the initially set alignment direction toward the alignment direction at driving by a very small angle, and they had not completely returned to the initial alignment direction, which caused decrease in contrast and gray scale reversal between adjacent pixels, thereby to degrade the image quality.
It is estimated from the above-stated facts that the after image and sticking image phenomenon which occur in the IPS mode are based on an after image mechanism specific to the IPS mode which is completely different from the mechanism in the conventional TN mode. It is necessary to solve the problem produced by a sticking image and afterimage specific to the IPS mode in which the liquid crystal is operated in the substrate plane to produce a display using the birefringence of the liquid crystal. Further, in the IPS mode of operation of a liquid crystal display device of the high transparent type in which movement of the liquid crystal on the electrode surface is also used for image display by employing transparent electrodes, the problem of after image also occurs at the same level or more severely. Therefore, a countermeasure against these problems of after image is required.